When Shannon Timpane separated from her ex-husband, she had two daughters, ages 1 and 3, a rickety truck, and no access to the couple’s bank accounts.

“I almost had a nervous breakdown,” she recalls. “I didn’t, because I had two little kids, and they were very cute and very sweet, and I didn’t want to ruin their lives.”

And she was grateful for at least one thing: She lived in California.

For single working moms like Ms. Timpane, experts say location is almost as crucial as in real estate. Timpane credits surviving those first few months to a variety of social programs available to support working mothers: She signed the family onto the state health insurance plan, relied on food stamps for four months, and spent a month with another family who paid her bills while she looked for work through the Adopt-a-Family program.

“California is a great state to be in for that kind of thing,” she says. “[It] has a lot more at its fingertips than most states do.”

Women with children under 18 now make up 70 percent of women in the workforce – up from 47 percent in 1975, according to the United States Bureau of Labor Statistics. The participation rate is even higher for single moms, about three-quarters of whom work.

But policies to help families vary greatly from state to state, studies show. New York, California, and Washington, D.C., ranked the highest in a 2015 study of the best and worst states for work and family, according to the Institute for Women’s Policy Research. Indiana, Utah, and Montana brought up the rear.

“We have a very uneven social safety net,” says Eve Weinbaum, director of the Labor Center at the University of Massachusetts in Amherst.

For example, only four states and the District of Columbia have laws that provide paid family leave – with California the first state to implement such a program in 2004.

Tami Holland is allowed 10 days of paid sick leave a year and three paid “personal days.” The single mother from Sterling, Va., who teaches special-needs children in an elementary school, prides herself on never having missed a school event or medical appointment for her daughter.

But that has sometimes meant forgoing a day’s pay.

“She’s my first priority, and I don’t want to miss those things,” says Ms. Holland, who had been a stay-at-home mom for 10 years until her divorce in 2011.

Like most working single parents, she is struggling against a view of the “traditional family” that is increasingly out-of-date. Married heterosexual couples with children under 18 now constitute just 20 percent of American households, down from 40 percent in 1970, according to US Census data.

Though the trend has been evident for decades, the social infrastructure to support families has remained largely the same, says Andrea Paluso, executive director of Family Forward Oregon, a group advocating for more support for working families.

“Most public policy is still designed around the idea that most families are structured with a married, two-parent household,” she adds. “Before, we didn’t have laws about access to sick time because the idea was somebody would be home with the kids and they didn’t need to be in the workforce, and now that’s not true.”

Timpane works two part-time jobs, one at a hospital and one at a local community college. She also is going to nursing school. When her children were younger, she worked night shifts so she could look after them in the afternoons.

Now she works during the day and often relies on friends, family, and other parents to give them rides. For one summer, 12 members of her book club even had to cover the costs of preschool. The bill for three hours of pre-school: $740 a month.

In 23 states, child care costs more than a four-year college education, according to a recent report from the Economic Policy Institute.

“The price of childcare is critical,” says Paula England, a sociology professor at New York University. “If the cost of child care is too high, you can’t pay for anything after childcare.”

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Like many single moms, Timpane says every dollar she earns is spent.

“You’re not saving for retirement, because you don’t have a future,” she says. “I’ll graduate nursing school and will be able to save money and put away for retirement, but it won’t be the same as someone who’s been saving since they’re 25.”

The consequences of dropping in and out of the workforce, working lower-paying jobs, and postponing saving for retirement can be significant. More than 1 in 7 women were living in poverty in 2014, according to the National Women’s Law Center, and more than two-thirds of the elderly poor were women.

“All of this stuff just adds up over the course of your lifetime, to the point where a vast majority of people living in poverty are women,” says Ms. Paluso.

At one point, the US did have a national child care system. For four years during World War II, the federal government implemented a universal childcare program that distributed funds to states to build and maintain child care centers while women worked in war factories.

The program stopped in 1946. In 1971, President Richard Nixon vetoed the Comprehensive Child Development Act, a measure passed with bipartisan support in the Senate that would have formed a national network of child care centers.

“We had, and I think probably continue to have, a real ambivalence about the idea of mothers and work and what that would mean,” says Ms. Paulso.

The lack of paid maternity leave is a prime example, adds Professor Weinbaum of the University of Massachusetts.

“It makes it hard for women to stay in the workforce, it makes it hard for women to move ahead in their careers, and it makes it hard for women to stay home and enjoy their families while their children are young.”

Having some national standard for helping working mothers would help, she says.

“Just like we have a minimum wage at the federal level, we should have minimum family policies that do create that safety net that do allow parents to know they can keep working and can take care of their families.”